WASHINGTON – A spectacular implosion of support risks turning Donald Trump’s presidential campaign into a historic ruin, as one institutional pillar after another collapses around him a month before election day.

Election historians say it’s unprecedented.

With advance voting already underway, Trump is being trampled by a stampede of Republicans abandoning him. At least one-third of the party’s senators have now declined to support Trump, rescinded their support, asked him to resign, or declared they’ll vote for someone else.

As if that weren’t enough, his own running mate publicly scolded him this weekend. The anti-Trump Republicans now include about 17 senators, 24 representatives, several governors – and counting.

Add a former Republican nominee to that list. John McCain has withdrawn his already-tepid backing. The final straw for him, and apparently for many others, was the release of an old video where the billionaire bragged about grabbing women’s genitals without permission.

“I have wanted to support the candidate our party nominated. He was not my choice, but as a past nominee, I thought it important I respect the fact that Donald Trump won a majority of the delegates by the rules our party set,” the 2008 nominee said in a statement Saturday.

“But Donald Trump’s behaviour this week, concluding with the disclosure of his demeaning comments about women and his boasts about sexual assaults, make it impossible to continue to offer even conditional support for his candidacy… (My wife) Cindy and I will not vote for Donald Trump.”

Several elected members went a step further: They asked Trump to resign, and let running mate Mike Pence replace him. Pence, for his part, cancelled a campaign appearance Saturday; declared himself offended by Trump’s remarks; and suggested he would be looking for contrition Sunday night in what is now destined to be a memorable debate against Hillary Clinton.

Trump vowed to stay in.

“The media and establishment want me out of the race so badly,” he tweeted.

“I WILL NEVER DROP OUT OF THE RACE, WILL NEVER LET MY SUPPORTERS DOWN!”

Growing fissures in the party are now cracking wide.

The divisions predated Trump. The rift between the party’s leadership and its membership is among the dominant themes in American politics. The big-business-friendly, immigration-supporting, trade-promoting leadership has clashed repeatedly with a grassroots that tilts against immigration and trade. That split has already affected American governance, stalling legislative efforts like long-delayed immigration reform.

Enter the corkscrew-coiffed human jackhammer now cracking it apart.

Trump is as reviled in institutional Washington as he is popular with a vast swath of primary voters. In the watering holes of the nation’s capital, it’s common to hear Republicans denigrate their own nominee.

This split is now evident everywhere.

At rallies this weekend, party figures like Paul Ryan were heckled for not sufficiently supporting Trump; the congressional leader has expressed dismay over the remarks, but not officially withdrawn his backing. On Facebook, former secretary of state Condoleezza Rice was deluged with criticism for declaring: “Enough! Donald Trump should not be President. He should withdraw.”

An example of the negative comments directed at Rice include: “Unfollowed. You care more about political correctness than the American people.”

Is there a historical precedent for a rebellion like this, at this stage in an election? When asked that question by The Canadian Press, three political historians answered unequivocally.

“Not in the last 150 years,” replied Allan Lichtman of American University. He said the closest comparisons he could think of were the smaller group of lawmakers who didn’t endorse 1964 Republican nominee Barry Goldwater or 1972 Democratic nominee George McGovern. “But nothing like this,” he said.

Michael Kazin of Georgetown University agreed this differs from the 1964 and 1972 cases. He said the holdouts then announced their decision long before October. He noted that both candidates lost in historic landslides.

Trump is not expected to suffer anything like those routs. He’s behind in the polls, but he continues to have support from Republican base voters and more than 40 per cent of the general electorate.

Another political historian said what’s unique in this election year is the late-campaign rebellion.

Margaret O’Mara of the University of Washington said the closest similarities occurred before the Civil War. She said parties were more fluid back then, with new ones popping up during campaigns. Also, Abraham Lincoln’s successor Andrew Johnson was abandoned by the Republican party while he was in office, although he wasn’t a Republican himself.

But this one’s unique, she said.

“I cannot think of an October Surprise of this variety in modern memory, perhaps ever.”